1. Technical Field
The present invention relates to a ballast circuit used to ignite and operate a high intensity discharge lamp and, more particularly for igniting and operating a ceramic high intensity discharge lamp.
2. Description of Related Art
A high-intensity discharge (HID) lamp produces light by means of an electric arc between electrodes housed inside an arc tube of a transparent material such as fused quartz or alumina. The tube is filled with both gas and a dose of metal salts. The gas facilitates an initial strike or ignition of an arc. Once the arc is started, the arc heats and evaporates the metal salts. A plasma is formed which greatly increases the intensity of light produced by the arc and reduces power consumption. In typically 1 to 2 minutes, a low powered 70 W HID lamp warms up to produce its rated light output. When the HID lamp is initially cool, an ignition voltage of 4000 volts for instance is typically required to ignite the HID lamp. After ignition, the HID ballast provides alternating current to the lamp at low voltage, e.g. 20-200 Volts. The physical properties of an HID lamp typically determine the operating voltage across the HID lamp.
Reference is now made to FIG. 2 which shows a plan cross-sectional view of ceramic HID lamp 14, according to conventional art. Ceramic HID lamp 14 includes electrodes 20 extending at the proximal ends of electrodes 20 into an arc chamber 28 which is interior to an arc tube 26. Electrodes 20 connect electrically at the distal ends to the output of the ballast circuit supplying the lamp 14. Electrodes 20 pass through respective bores 24 of capillaries 25 and are sealed inside a portion of bore 24 by seal 22 near the distal ends of capillaries 25.
Lamp 24 of construction as shown in FIG. 2 is known as a “ceramic” HID lamp in distinction with a “quartz” HID lamp with arc tube of material fused silica or polycrystalline quartz. In quartz HID lamps, the seal to the electrode is formed by pinching the tube material while in a viscous semi-liquid state onto the electrode near the entrance to the arc chamber. In ceramic HID lamps of ceramic materials other than fused silica or quartz, the pinch seal is not available and seal 22 is formed by melting glass or ceramic frit inside distal portions of bore 24 within capillaries 25. U.S. Pat. Nos. 7,701,142, 7,728,495 and US patent application publication US20020145388 are representative references describing ceramic arc lamps.
Ceramic HID lamps 14 may provide improvements over the quartz metal halide (MH) lamps, both in the light efficacy, color temperature and color rendering index (CRI). Normally the color temperature of MH quartz lamps is over 4000 kelvin with CRI of 65 to 70. Ceramic lamps 14 typically may provide warmer light typically around 3200K with CRI of 90. Light efficacy may be over 110 lumen per Watt (L/W), while from quartz MH lamps the efficacy is typically around 90 L/W.
Most HID lamps, including ceramic HID lamps 14 are operated at low frequency of less than 400 Hz. However, operating at high frequency range, over 100 kHz may provide advantages such as longer life, lower lumen depreciation, stable color and CRI. For instance, lumen depreciation of a quartz metal halide (MH) lamp at low frequency operation after 8000 hours may go down as low as 50% of the initial value, while operating the same lamp at high frequency may show lumen depreciation of only less than 15% at 8000 hours of operation. Ignition with high frequency may improve even further HID lamp performance over life time comparing to the conventional low frequency ignition methods. However, it is known that high frequency operation on HID lamps may result in acoustic resonances. Acoustic resonance typically may cause a flicker of the light output. Above 100 kHz, most lamps may operate free of acoustic resonances. One known way to eliminate acoustic resonances in quartz HID lamps at high frequency may be by implementing low frequency modulation superimposed on the high frequency operation waveform.
Thus there is a need for and it would be advantageous to have a system and method for ignition and operation of ceramic HID lamps 14 at high frequency reliably over a long lifetime with minimal deterioration of performance and without acoustic resonance.
The term “bore” 24 hereinafter refers to the hollow portion interior to capillary 25 and proximal to seal 22. The term “proximal” herein refers to the ends of electrodes 20 and/or capillaries 25 which protrude into arc chamber 28. The term “distal” as used herein refers to outer ends of electrodes 20 and/or capillaries 25 more distant from the center of arc chamber 28.
The term “peak voltage” as used herein refers to absolute value peak voltage.
The term “high frequency” as used herein in the context of high frequency normal operation of a HID lamp refers to an operation around or greater than 100 kiloHertz.
The term “low frequency” as used herein in the context of low frequency normal operation of a HID lamp refers to an operation at a frequency of order of magnitude or less than 400 Hertz.